1. Field of the Invention
The present invention relates to a printing head of a wire-dot impact printer.
2. Description of the Related Art
Recently, a high-speed printing capability of a wire-dot impact printer has been accompanied by the use of a laminated piezoelectric device for driving of printing wires provided in a printing head. The laminated piezoelectric device includes a plurality of internal electrodes and a pair of external electrodes for connecting the internal electrodes, and it is driven at a high voltage under severe conditions to therefore require a high reliability.
A wire-dot impact printer employing such a piezoelectric device is described in U.S. Pat. No. 5,005,994, for example, issued Apr. 9, 1991 to Akio Yano. A printing head of the wire-dot impact printer described in this patent is constituted of a frame, a plurality of impact printing wires constituting a wire-dot matrix, and a plurality of actuators for respectively and selectively driving the impact printing wires.
Each actuator is constituted of a displacement enlarging mechanism, a laminated piezoelectric device, a base for connecting the displacement enlarging mechanism to the laminated piezoelectric device, and a movable block fixed to one end of the piezoelectric device. The displacement enlarging mechanism is constituted of an armature and a pair of leaf springs disposed in parallel relationship to each other for supporting one end portion of the armature. The piezoelectric device is fixed at its other end to the base. One of the leaf springs is connected between the armature and the movable block, and the other leaf spring is connected between the armature and the base. The impact printing wire is fixed to the other end portion of the armature.
In printing, a driving voltage is applied to the piezoelectric device to generate a strain of tens of .mu.m in the piezoelectric device and thereby push the movable block. As a result, the armature is swung about a lower end portion thereof by a bimetal effect of the pair of leaf springs, and the displacement of the movable block is therefore enlarged to project the printing wire in an amount of hundreds of .mu.m. Then, the printing wire is impacted against a platen through an ink ribbon and a printing paper to form a dot print on the printing paper.
FIG. 1 shows a shape and a stress distribution of each leaf spring 2 used in the prior art printing head. As apparent from FIG. 1, the shape of the leaf spring 2 is rectangular, and the distribution of a stress generating in the leaf spring 2 in operating the piezoelectric device is such that the stress is very high at the opposite ends of the leaf spring 2 and it is low at the central portion of the leaf spring 2. As mentioned above, one of the leaf springs is fixed at its opposite ends to the armature and the movable block, and the other leaf spring is fixed at its opposite ends to the armature and the base. Accordingly, when the piezoelectric device is driven, the stress generating at the opposite ends of each leaf spring becomes very high to cause a defect that each leaf spring is broken at the fixed portions. The breakage of the leaf springs occurs at the fixed portions to the movable block and the base more frequently than at the fixed portions to the armature.